1. Field of the Invention
The present invention relates to angle sensors, and more particularly to an angle sensor in which a magnet and a magnetic detector are disposed such that they face each other so as to detect the rotational angle of the magnet relative to the magnetic detector based on an output of the magnetic detector.
2. Description of the Related Art
In a typical known angle sensor, as shown in FIG. 10, a magnetic detector 2 is disposed such that it faces a rectangular magnet 1 which is attached to a rotational shaft (not shown) by allowing center O to coincide with the core of the shaft, and the rotational angle of the magnet 1 is detected based on an output of the stationary magnetic detector 2.
A magnetic detector is a Hall device or a magnetic resistor, which generates an output voltage or changes its resistance, respectively, in proportion to the magnitude of the magnetic flux density vertically passing through the device. In the case of the Hall device, for example, when the circumferential position of the Hall device relative to the magnet 1 changes from A, B, C, D, and E, the output voltage is changed, as shown in the diagram of FIG. 11.
In this type of angle sensor, in order to improve the detection precision, it is preferable that the relationship between the angle and the output is linear over the entire range of the angles to be detected. It is also preferable that the detectable angle range is wider. Accordingly, it is necessary that the output linearity of the magnetic detector be improved and the electrical effective angle range be increased.
Details of the output linearity and the electrical effective angle range of the magnetic detector are provided below. When the magnetic detector 2 is located at position A relative to the magnet 1, it is positioned in proximity to the S pole of the magnet 1. When the magnetic flux from the N pole is positive, the negative magnetic flux density becomes maximum in the magnetic detector 2, and the output voltage becomes minimum. Conversely, when the magnetic detector 2 is located at position E, it is positioned in proximity to the N pole, and accordingly, the output voltage becomes maximum. When the magnetic detector 2 is located at position C, the magnetic flux orthogonal to the magnetic detector 2 is zero, and thus, the output voltage becomes zero. In this specification, position C is referred to as the “neutral detection position”, and when it is detected that the magnetic detector 2 is located at position C, this state is referred to as the “neutral detection state”.
The ideal output characteristic is indicated by the straight line X shown in FIG. 11, and FIG. 12 shows a deviation of the output characteristic from the straight line X, which is indicated by the ratio to the maximum variable voltage (in this case, 0.15 V) of the detection range (in this case, ±60°). A smaller deviation is preferable in order to achieve the linear relationship of the output to the angle.
The electrical effective angle range includes the angles that can be detected while maintaining the output linearity. The effective angle range varies according to the purpose for the use of the sensor, and a wide range of angles is not always needed for all the purposes. However, although a sensor having a wide range of effective angles can be used in a narrow range of angles, a sensor having a narrow range of effective angles cannot be used in a wide range of angles. Accordingly, a sensor having a wider range of electrical effective angles is more practical. The electrical effective angle range of a known angle sensor exhibiting the characteristic shown in FIG. 11 is ±60°.
Previously, the magnet is formed in the shape of a rectangular parallelepiped in view of high manufacturability, and the rectangular-parallelepiped magnet generates the magnetic flux shown in FIG. 13. When the magnet 1 is rotated around the center O, as shown in FIG. 10, the gradient of the output voltage between positions B and D is determined by length L. The output voltage of the maximum magnetic flux density of the S pole at position A and the output voltage of the maximum magnetic flux density of the N pole at position E are determined by the distance between the magnetic poles and the magnetic detector 2 (radius r), and the output level (magnetic flux density) is increased with a smaller distance therebetween.
However, the output characteristics at positions A and E (±90°) is slightly curved, and the effective angle range that can maintain the output linearity is at most about ±60°.
Japanese Unexamined Patent Application Publication No. 2000-121309 discloses the following angle sensor shown in FIGS. 14A and 14B that improves the output linearity of the magnetic detector. A semicircular magnet 5 is inserted into the side groove of a rotational shaft 6 such that center P becomes eccentric with respect to center O of the rotational shaft 6, and a Hall device 7 is fixed such that it faces the magnet 5.
In the angle sensor disclosed in the above-described publication, the magnet 5 is formed to be semicircular so as to make the magnetic pole portions acute. Accordingly, the magnetic flux density is biased to the center O (see FIG. 15), thereby improving the linearity of the output characteristics within the effective angle range. However, the effective angle range is at most about ±60°, and cannot be increased further than that. Additionally, the manufacturing cost is increased because the magnet 5 has to be formed to be semicircular.